A polycarbonate generally contains bisphenols as a monomer component, and taking advantage of its superiority in transparency, heat resistance, mechanical strength and the like, the polycarbonate is widely utilized as a so-called engineering plastic, for example, in the fields of electrical/electronic part and automotive part and in the optical field such as optical recording medium and lens.
The conventional polycarbonate is produced using a raw material derived from a petroleum resource, but in recent years, the depletion of petroleum resources is concerned, and it is demanded to supply a polycarbonate using a raw material obtained from a biomass resource such as plant. Also, to avoid the fear that the global warming caused by an increase in the carbon dioxide emission and an accumulation thereof may bring about climate change, it is demanded to develop a polycarbonate starting from a plant-derived monomer that is carbon-neutral even when discarded after use.
Under these circumstances, there has been proposed a method where isosorbide (ISB) that is a dihydroxy compound obtained from a biomass resource is used as a monomer component and a polycarbonate is obtained through transesterification with a carbonic acid diester while distilling off a monohydroxy compound byproduct under reduced pressure (see, for example, Patent Documents 1 to 7).
However, a dihydroxy compound such as isosorbide (ISB) has a low boiling point as compared with bisphenols and therefore, its significant volatilization takes place during the transesterification reaction performed at a high temperature under reduced pressure, giving rise to a problem that not only the raw material consumption rate is worsened but also in the case of using a plurality of kinds of dihydroxy compounds, the molar ratio of the dihydroxy compounds used is changed during polymerization to make it impossible to obtain a polycarbonate having a desired molecular weight or composition.
In order to solve such a problem, there have been proposed a method of performing the reaction under an ordinary pressure in the initial stage of the reaction, thereby consuming the monomer and preventing its volatilization, and a method using a polymerization reactor with a specific reflux condenser (see, Patent Document 8).
However, the studies by the present inventors have revealed that the conventional method involves worsening of the color tone of the polycarbonate obtained. This is because the transesterification reaction is an equilibrium reaction, in other words, the reaction is accelerated by removing the reaction byproduct from the reaction system, and when the volatilization of the monomer is suppressed at the initial stage of the reaction, the reaction rate is inhibited at the same time, as a result, the heat history applied to the reaction is increased and furthermore, a thermal decomposition product stays in the reaction system. In particular, a polycarbonate starting from the dihydroxy compound having a specific structure of the present invention is poor in the thermal stability as compared with the conventional polycarbonate starting from bisphenols and therefore, the worsening of the color tone is outstanding.
Also, the transesterification method using diphenyl carbonate as the carbonic acid diester involves generation of a large amount of a phenol byproduct and as the method for treating the byproduct phenol, there has been proposed a method of recovering the byproduct phenol by distillative purification and reutilizing it as a raw material of diphenyl carbonate or bisphenol A (see, for example, Patent Document 9).